Debug Info: for static member variables, move AT_MIPS_linkage_name from
TAG_member inside a class to the specification DIE.

Having AT_MIPS_linkage_name on TAG_member caused old gdb (GNU 6.3.50) to
error out. Also gcc 4.7 has AT_MIPS_linkage_name on the specification DIE.

rdar://problem/13291234